This invention relates to a clock reproducing method, in particular, to a receiving clock producing method which enables real time transmission of a moving picture signal on a network, such as a packet switching network or an asynchronous transfer mode (ATM) network, by reducing or absorbing jitter on a received data signal.
In the network such as the packet switching network or the ATM network, the jitter is apt to occur on a transmitted signal transmitted through a transmission channel of the network. In addition, the network does not guarantee to establish network synchronization between a transmitting side and a receiving side. Accordingly, the network has a problem that delay time of the received data signal has fluctuation. That is, the received data signal has the jitter given by the transmission channel. To reduce or absorb the jitter on the received data signal, a jitter-absorbing buffer is necessary at the receiving side.
Herein, the term of “jitter” represents a state that pulses of the received data signal deviate forward or backward from normal time positions. That is, the pulses of the received data signal has phase variation or fluctuation in this state. Accordingly, the state may be considered that undesirable phase shift modulation is provided to the transmitted signal. The undesirable phase shift modulation appears as jitter noise when the received data signal is demodulated. Thus, the jitter deteriorates transmission quality of the network.
The jitter is roughly classified into two types. One type is transmission channel jitter caused by transmitting the transmitted signal through the transmission channel. The other is a stuff jitter brought by pulse staffing synchronization for multiple synchronization. This invention aims to reduce or absorb the transmission channel jitter.
Generally, a receiving buffer different from the jitter-absorbing buffer is provided in the receiving side. To prevent the receiving buffer from overflowing, a reading clock (or a receiving clock) must be synchronized with a writing clock (or a transmitting clock). Thus, a related receiving clock producing apparatus adopts an adaptive clock reproducing method that its notion is defined by ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) recommendation I.363.1. According to an existing adaptive clock reproducing method, the amount of data staying in the receiving buffer is monitored and the receiving clock is controlled so that the amount of the data staying in the receiving buffer is equal to a predetermined volume.
The existing adaptive clock reproducing method does not consider the transmission channel jitter. Thus, it is hard to reduce or absorb considerably the transmission channel jitter on the received data signal by the use of the receiving buffer. Therefore, the related receiving clock producing apparatus provides the jitter absorbing buffer mentioned above. That is, the related clock reproducing apparatus has two buffers and is complex in structure.